This invention relates to radiologicial well logging methods and apparatus for investigating the lithological characteristics of subsurface earth formations traversed by a borehole and, more particularly, relates to improved neutron-gamma ray logging methods and apparatus.
It is well known that oil and gas are more likely to be found in commercially recoverable quantities in those earth formations which are relatively porous and permeable than in the more highly consolidated formations. It is also well known that an oil or gas-filled strata may be located by passing a neutron source through the borehole and measuring the intensity of secondary gamma radiations which are produced at various depths in the borehole. A chlorine nucleus has a thermal neutron capture cross section which is much higher than that of nuclei of most of the other elements which are found in greatest abundance in the earth, and thus a salt water filled limestone or sandstone layer will have a greater macroscopic thermal neutron capture cross section than will an oil saturated layer. Accordingly, this difference can be observed by measuring either chlorine capture gamma rays or the lifetime of the thermal neutron population in the layer.
Although these logging techniques have long been used, and although a great many oil or gas-bearing formations have been found in this manner, they have also produced a great many spurious indications. This is because many porous earth formations contain low salinity water, which is indistinguishable from oil using these methods. Thus, the intensity of the capture gamma radiation which is detected at various borehole depths is an indication of fluid salinity and porosity, and is not necessarily conclusive evidence that an oil-bearing formation has been discovered. An inelastic gamma ray spectrum, however, is independent of salinity since chlorine has a small inelastic cross section.
The carbon nuclei in the oil will, to a limited extent, also engage in capture interactions with bombarding neutrons, although the thermal neutron capture cross section of carbon is extremely low, and this is also true for oxygen nuclei. However, the inelastic scattering reaction cross section is appreciable for both carbon and oxygen if the collision energy of the neutron is sufficiently high. Furthermore, the initial energies of the gamma rays resulting from carbon are distinctively different from that of gamma rays resulting from oxygen when this reaction occurs. Accordingly, it has long been assumed that a measurement of inelastic scattering gammas could provide the basis of a technique for detecting and identifying an oil or gas-bearing earth formation as opposed to a water-bearing formation as such.
Many attempts have been made to employ this concept in well logging. Thus far, however, none of the methods and apparatus which utilize this concept have been reliable.
One of the principal reasons for this lack of success is that carbon is one of the most common elements in the earth's crust. Moreover, a limestone formation is largely composed of calcium carbonate, and thus a water-bearing limestone formation will frequently emit more carbon gammas than will an oil-filled sand or shale.
Another problem is that a gamma ray tends to readily engage in scattering reactions itself and further tends to lose energy to some extent with each scattering. The initial energy of a gamma which results from the inelastic scattering of a neutron by an oxygen nucleus is only a little higher than the initial energy of a gamma resulting from inelastic scattering of a neutron by a carbon nucleus. Thus, many of the gammas which emanate from fast neutron bombardment of a water-bearing formation will frequently have declined in energy by the time they are actually detected, whereupon they may be mistaken for non-degraded gamma rays which have emanated from carbon nuclei.
Not all oxygen-emitted gamma rays will be degraded before they are detected, however, and thus it has been proposed to log a well with the detector signal being applied to a "two-window" analyzer. More particularly, one window is set to accept only pulses attributable to detected gammas having terminal energies which approximate the initial energies of oxygen-emitted gamma rays, and the other window is set to accept only pulses comparable to gammas with terminal energies corresponding to carbon-emitted gammas. Thus, the counting rates of the two windows may be compared to provide an indication of whether a particular formation contains oil or water.
Although such a log has been performed with some success, it nevertheless is often unreliable and has therefore never been universally accepted by the petroleum industry. A principal reason for its lack of reliability is, again, the fact that gamma rays tend to quickly lose energy, and this is especially true when the gamma radiation encounters a relatively dense medium. Thus, a high count rate for the lower of the two windows may actually be due to the fact that most of the oxygen-emitted gamma rays have become degraded before detection, by reason that the gammas were required to pass through the formation, the liquid-filled borehole, the sonde case, and possibly a cemented casing before they could reach the detector in the logging tool. Furthermore, most oil-bearing formations also contain at least an appreciable amount of water, and the oil/water interface is extremely difficult to detect merely by a qualitative measurement of the number of carbon-emitted gammas which manage to reach the detector.
Many attempts have been made to improve the foregoing technique. For example, the degrading effect of the drilling fluids in the borehole has been reduced by decentralizing the logging instrument in the borehole. Also, the size of the phosphor used in the detector has been increased substantially in order to sense a greater proportion of the gamma rays sought to be detected, whereby the measurement has been improved from a statistical standpoint. Although most if not all of these changes have been of some benefit, no change has been found which would make any log fully acceptable to the petroleum industry which is based principally on this concept.
These and other disadvantages of the prior art are overcome with the present invention, however, and novel well logging methods and apparatus are provided for simultaneously and correlatively measuring the gamma radiation resulting from the inelastic scattering of neutrons by carbon and oxygen nuclei as a function of the lithological characteristics of a subsurface earth formation.